# Batteries Discharge and Recharge time.

Discussion in 'Power Electronics' started by Moha99, Mar 29, 2012.

1. ### Moha99

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Nov 18, 2011
Hey all, I was thinking about this a few days ago... does recharging a battery takes a longer time than discharging it?

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Not necessarily. But it does take more energy.

3. ### Moha99

261
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Nov 18, 2011
Yea I knew that for a fact but if I use a battery for like 10 mins. then recharge it with a little more power you think it recharges in 10 mins? How can I calculate this process perfectly?

And if i recharged the battery with less power would it take longer to recharge?

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Charging and discharging are independent in this respect.

If your maximum discharge current exceeds your maximum charge current then you can discharge your battery far more quickly than you can charge it (and this is typically the case)

it really depends on the discharge current vs the charge current and the efficiency of the charging process. The fact that charging can be non-linear (the charge current tapers off at the end of the charge) further complicates things.

5. ### cjdelphi

1,096
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Oct 26, 2011
Moha, the more energy you push into the battery the more heat produced, the heat produced is simply wasted energy, so slower the charge the better and the battery will last longer as a result.

eg, if a battery is (1.2v 1000mah ) (with exception to the tapering off near full charge) if completely drained, you can calculate how long you need to charge it for...

eg, if you deliver a constant voltage/ constant current.. 100ma/1000 = 10 hours to fully charge) but, if you put in 1000ma you'd have it charged in 1 hour and so on.

but.... If most intelligent chargers, will pulse charge, eg 1000ma for 2/3 seconds then it stops charging, measures the voltage (if it's at 1.5v stop, if it's less than 1.5v keep charging) etc, 4.25 for lithium, different values for different battery types..... ni-cad, lithium-ion, li-poly, nimh, and all the rest... so it purely depends on what the battery is and what it was designed for..

bottom line... if you over charge, or put too much current through, the thing will if you're lucky just explode.... lithium if you're lucky will only burn 1 room down

6. ### Moha99

261
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Nov 18, 2011

Wow thanks for that advice! Really appreciate it now i can avoid hurting my self and causing damages in my room!

But on thing. If I draw for example 1.2V at 2.5Amps how long would you think the battery will last?

And since i can't give the battery more energy for safety reasons I'll charge it with the exact same amount of current so if I used it for 10 HOURS I'll recharge it for 10 HOURS?

So it's balanced right?

7. ### GreenGiant

842
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Feb 9, 2012
keeping with the 1.2v/1000mAh drawing 2.5 amps will drain the battery completely (under perfect conditions) in 24 minutes (60mins/2.5)

you can charge a battery with more current than is drawn from it, you just have to be careful with it, again with the example 1.2V/1000mAh drawing at 100mA will yield a 10 hour discharge, you can safely charge it at 1000mA (1A) and it will be charged in 1 hour

a general rule of thumb is the rated c-rate (as it is called... 1000mAh) is what you can safely charge it at, though with most cells you can double that rate, and sometimes all the way up to 10 times that rate, though it is recommended to stay close to the noted c-rate.

8. ### Moha99

261
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Nov 18, 2011
The battery is rated at 1.2V 2500mAh. I'll draw about 1.2V and 2500mAh and then recharge it with 1.2 and 3amps.

And btw im building a charging unit with its own circuit the power is about 30W and im afraid im giving the battery more power than it needs.

30/1.2V = 25Amps and that a lot to give...Im worried that i would supply much more power to the batteries what should i do in this case?

261
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Nov 18, 2011
... Anyone?

10. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Discharging at 2.5A will discharge the battery in under an hour. You also won't get 1.2V all the time, so specifying the voltage at which you are discharging is pointless.

Charging at 3A will charge it in about an hour.

I hope you know when to terminate the charge because if you don't you may damage the cell.

edit: the current is limited by the charge on the batter and its internal resistance. Unless you're doing something really silly (or amazingly well optimised) you won't get anything near that current.

How do you detect end of charge? If you''re rapid charging a cell (which, incidentally often does not fully charge the cell) then you need to be very careful about overcharging.

Last edited: Mar 31, 2012
11. ### Shyamal796

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Mar 19, 2012

It take longer to charge.

12. ### Moha99

261
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Nov 18, 2011
So the charging and the discharging is not equally balanced? Even if i discharge it at the same rate as recharging it?

Im discharging it at a certain rate and recharging it with more energy...

bottom line is: can there be a way to maintain a balanced re-charge/discharge rate?

And if not could that be possibile throughout super-capacitors? Even if it was difficult I'd like to try it out.

13. ### Moha99

261
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Nov 18, 2011

I think that depends on how we use it and discharge it.

14. ### Moha99

261
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Nov 18, 2011
I do detect the end of charge i will try to monitor it all the way and I'd like to make a perfect mathematical formula to depend on it.

15. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
I'm sure there was a post in this thread where you were told that you need to put more energy into a battery to recharge it than you take out when you discharge it.

You charge a battery until it is charged, then you stop. And depending on the battery type, the rate at which you charge it may alter one or more times during the process.

A battery is not a capacitor. And even if it were, there are still losses.

16. ### Moha99

261
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Nov 18, 2011
Fair enough! Then more power it is.

Are there any formula's or laws that i can use to build my own formula of charging and discharging? that i need "MORE ENERGY" point is their a specific amount of energy needed to satisfy the charging process?

If possibile someone could right down some laws or anything that i can lean to when I'm working on charing and discharging... with more details if possibile.

Thanks!

17. ### davennModerator

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Sep 5, 2009
there's no general laws for that. It depends on all the factors relating to the type of battery.
Say if it's a li-po, a lead acid, a nickel metal hydride, a Nickel Cadnium. Etc etc .
Each have their specific charging requirements.... Some are constant voltage, some are constant current, some are variable current. Most require specific charge rates for specific times. Some can be trickle charged contineously, some cant. Some can be hi current fast charged some can't, else they can burst into flames or explode. You need to do some google searching on the specific types of batteries you are interested in working with to find out how they need to be charged.

Cheers
Dave

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Nov 18, 2011
19. ### Moha99

261
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Nov 18, 2011
My goal is to charge to a battery at a shorter time than discharging it Steve told me since im using about 2.5Amps in one hour i should re-charge it with 3Amps and it would take an hour.

The batteries are rated at 2.3 amps Im going to try and charge then with 3Amps since im building the charging unit... No worries I will not OVER charge it as soon as calculate exactly how long is the re-charge time I'll stop charging it at once.

20. ### davennModerator

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Sep 5, 2009
A bit unfortunate that the data sheet doesn't give recommended charging info
But I will tell you now, if you intend to continuously fast charge and discharge
Don't expect the batteries to have a long life till they fail

That's from experience, using fast chargers out in the field to charge same type of battery when doing storm chasing filming/ photography

Dave

Dave